This invention is directed to a method for inhibiting corrosion and the deposition of scale and sludge on heat-transfer surfaces of cooling water systems at cooling water temperatures below 212.degree. F. The sludge formed from the hardness of water of salts etc. in cooling water is conditioned to prevent adherence to the internal metal surfaces of the cooling systems, and the particle size of the sludge is maintained sufficiently small, in accordance with this invention, to keep the sludge in suspension with at least one polymeric dispersants for removal by side-stream filtration. In water cooling systems and particularly in recirculating systems such as cooling towers and the like, there is a tendency for the insoluble salts, e.g., inorganic alkaline earth metal salts to deposit on the heat transfer surfaces. In accordance with this invention, these salts are conditioned by treating the cooling water with an effective amount of hydrogen peroxide, water soluble polymers derived from carboxylic acids such as polymers of maleic acid or acrylic acid, a non-ionic surfactant and at least one triazole corrosion inhibitor. It has been found, that in addition to inhibiting the insoluble salts from depositing on the cooling surfaces, the treatment of the water inhibits crystal growth and prevents adherence to the metal surfaces by conditioning the heat transfer surface's ability to absorb the crystals, and thereby impedes the buildup of scale and the like.
The formation of scale on heat transfer surfaces is a problem in heat-exchange or cooling-water equipment in power stations, industrial cooling systems, and the like. Scale and other inorganic metal deposits lower the efficiency of heat transfer, causing overheating and damage to the equipment. Deposits from cooling water comprises crystalline scale, metal oxides, carbonates, etc., which precipitate on the internal surfaces of the equipment. Scale is formed primarily by crystallization when the scale-forming constituents become super-saturated and deposit from solution onto the metal surface. Many factors influence scale formation, including solubility of the various materials such as calcium carbonate, calcium sulfate, and the like, and the tendency of these materials to become super-saturated as water evaporates from the systems. The formation of scale in water cooling, i.e. at temperatures below 212.degree. F. can be controlled, however, by chemical treatment, not only to inhibit the formation of scale but also to remove and prevent the accumulation of scale on the cooling surfaces.
Moreover, because of the increase in cost of supplying makeup-water for the cooling system coupled with the rising expense for waste-water treatment, there have been attempts to reuse the waste water streams as the cooling water makeup. Reused water can include various water sources such as blowdowns, processed water, plant discharge water and the like. The reuse of the water requires the recycling of the cooling tower blowdown back to the cooling systems. One of the major incentives for reusing cooling-tower blowdown water is the high cost of removing the toxic additives such as chromates etc., from the water prior to discharge.
Presently, various scale inhibitors are being used to prevent precipitation and adhesion of the scale to the metal surfaces. These inhibitors include such compounds as the inorganic phosphorous compounds, e.g., the phosphoric acid salts, the organic phosphorous compounds, like the alkyl phosphates and alkyl phosphites and various organic polymers. To prevent corrosion, known inhibitors such as the chromates are being used, which are known to be dangerous to the environment and therefore present environmental problems. To overcome and avoid the use of toxic chemicals such as chromates, various polyphosphoric acids and the salts thereof, e.g., phosphonic acid and the phosphoric acid esters are being used as a replacement. In addition, various other materials including the lignins, starches, tannins, etc., have been used as conditioners for sludge without satisfaction.
A number of patents disclose the use, for example, of polymers of styrene-sulfonic acid with other polymerizable monomers such as maleic acid for water treatment, see U.S. Pat. No. 4,048,066. U.S. Pat. No. 3,671,448 also discloses the use of phosphonic acids for the treatment of scale, and the use of these acids in combination with other water treatment chemicals. Further, U.S. Pat. No. 3,297,578 discloses the use of diphosphonic acids for purposes of inhibiting incrustations while U.S. Pat. No. 3,289,734 discloses a scale-inhibiting process using copolymers of styrene and maleic anhydride. U.S. Pat. No. 4,217,216 discloses the use of phosphonic acid derivatives in combination with azoles and molybdates for inhibiting corrosion and scale. However, none of these prior art patents suggest the process of treating cooling water at temperatures below 212.degree. F. as set forth in this invention.